1. Field of the Invention
The present invention relates to a surface acoustic wave device, such as a surface acoustic wave filter or a surface acoustic wave duplexer (demultiplexer), and a composite module including the surface acoustic wave device.
2. Description of the Related Art
A surface acoustic wave (SAW) filter is a filter utilizing characteristics of a surface acoustic wave (SAW), i.e., a wave propagating along a substance surface. A surface acoustic wave (SAW) duplexer is a duplexer using the surface acoustic wave filter. In surface acoustic wave devices such as the surface acoustic wave filter and the surface acoustic wave duplexer, an IDT (Interdigital Transducer) is formed on a piezoelectric substrate by employing a metallic thin film. In that type of surface acoustic wave device, electrical inputting and outputting are performed by exciting and receiving the surface acoustic wave based on the piezoelectric effect. The surface acoustic wave device functions as a filter through a process of selecting only a frequency component f that satisfies the relationship of f=v/λ where v denotes the propagation velocity of an acoustic surface wave depending on a substrate material, and λ denotes a period of electrode fingers of the interdigital transducer. In comparison with the propagation velocity of an electromagnetic wave, i.e., about 300,000 km/s, the propagation velocity v of the surface acoustic wave in the surface acoustic wave filter is very slow, i.e., 2000 m/s to 5000 m/s, and the period λ of the electrode fingers in the surface acoustic wave filter is small. Therefore, the filter utilizing the surface acoustic wave is suitable for downsizing and is widely used particularly in mobile communication devices, such as a cellular phone, in recent years.
In many of those mobile communication devices, electronic components are used in the form of modules. Japanese Unexamined Patent Application Publication No. 2008-227748 discloses a surface acoustic wave device in which deformation of a hollow structure of the surface acoustic wave filter caused by pressure applied in forming the module is suppressed, and a method of manufacturing the surface acoustic wave device.
FIG. 6 is a sectional view of a surface acoustic wave device, as an electronic component, disclosed in Japanese Unexamined Patent Application Publication No. 2008-227748. An interdigital transducer 16 is disposed on one principal surface 12 of a piezoelectric substrate 10. An insulating layer 18 is disposed on the one principal surface 12 around the interdigital transducer 16, and a cover layer 20 is arranged over the insulating layer 18 and the interdigital transducer 16 so as to cover them. Thus, to maintain the specific function, the surface acoustic wave device has a hollow structure in which a cavity is formed above the interdigital transducer 16 which is a vibrating portion, and the surroundings of the interdigital transducer 16 are sealed off by the insulating layer 18. Furthermore, the surface acoustic wave device illustrated in FIG. 6 includes a via hole electrode 22 penetrating through both the insulating layer 18 and the cover layer 20 and connected to the interdigital transducer 16 through a connection wiring 21, and a connection electrode 24 arranged at an end portion of the via hole electrode 22 on the side opposite to the piezoelectric substrate 10.
In the case of a surface acoustic wave filter of transversal type, for example, the interdigital transducer 16 is constituted by an input interdigital transducer 16a and an output interdigital transducer 16b. When an electric signal is applied to the input interdigital transducer 16a, a surface acoustic wave corresponding to the period of the electrode fingers is generated based on the piezoelectric effect. The generated surface acoustic wave is propagated to the output interdigital transducer 16b along the surface of the piezoelectric substrate 10. A signal output having a frequency corresponding to the period of the electrode fingers of the output interdigital transducer 16b as description above.
Recently, reductions in size and thickness of a composite module incorporating the above-described surface acoustic wave device have been demanded eagerly, and the necessity of reducing the height of the surface acoustic wave device has been increased correspondingly.
In trying to reduce the thickness of the surface acoustic wave device, it is also required to reduce the thickness of a piezoelectric substrate. However, when the thickness of the piezoelectric substrate 10 is reduced, a bulk wave generated together with a surface acoustic wave causes a problem with filter characteristics. More specifically, when a signal is input to the input interdigital transducer 16a, the bulk wave propagating inside the piezoelectric substrate 10 together with the surface acoustic wave is radiated in the direction of thickness of the piezoelectric substrate 10. The radiated bulk wave is reflected at the other principal surface 14 of the piezoelectric substrate (i.e., a principal surface of the piezoelectric substrate where the interdigital transducer 16 is not formed), and is superimposed on the surface acoustic wave in the principal surface 12 of the piezoelectric substrate on the same side as the interdigital transducer 16. In the case of a thick piezoelectric substrate, although the bulk wave is reflected at the other principal surface 14 of the piezoelectric substrate, the bulk wave is attenuated because a propagation length up to the principal surface 12 is relatively long. In the case of a thin piezoelectric substrate, however, the propagation distance is shortened and an attenuation of the reflected bulk wave inside the piezoelectric substrate is reduced. Hence, an amount of the bulk wave reaching the principal surface 12 is increased and an influence upon the surface acoustic wave is also increased.